1. Field of the Invention
The present invention relates to an exposure apparatus for transferring a circuit pattern drawn on a mask onto a substrate coated with a photosensitive agent and, more particularly, to an exposure apparatus which uses a laser as a light source and with which good exposure amount control is possible when the oscillation wavelength of the laser source is to be changed.
2. Description of the Related Art
Conventionally, in the process of manufacturing a semiconductor element, e.g., an LSI or a VLSI, which is formed of a very fine pattern, a reduction projection exposure apparatus for reducing and projecting a circuit pattern drawn on a mask onto a substrate coated with a photosensitive agent and printing it is used. As the integration density of the semiconductor elements increases, a finer pattern is required. Along with a progress in the resist process, the exposure apparatus must cope with a finer pattern.
As a means for improving the resolving power of the exposure apparatus, a method of changing the exposure wavelength to a shorter one and a method of increasing the numerical aperture (NA) of the projection optical system are available. It is generally known that a resolving power is proportional to the exposure wavelength and inversely proportional to the NA. Attempts have been made to keep the depth of focus of the projection optical system while improving the resolving power. Generally, the depth of focus is proportional to the exposure wavelength and decreases to be in an inverse proportion to the square of the NA, and to improve the resolving power and to keep the depth of focus are contradictory issues. In order to solve this problem, the phase shift reticle method, the FLEX (Focus Latitude Enhancement Exposure) method, and the like are proposed.
Regarding the exposure wavelength, a KrF excimer laser having an oscillation wavelength near 248 nm has become the mainstream recently to replace a 365-nm i-line. As the next-generation exposure light source, an ArF excimer laser having an oscillation wavelength near 193 nm is under development.
From the viewpoint of the manufacturing cost of the semiconductor element, a further increase in throughput in the exposure apparatus has been made. For example, a method of shortening the exposure time per shot by increasing the output from the exposure light source, a method of increasing the number of elements per shot by increasing the exposure area, and the like are proposed as the attempts aiming at this increase in throughput.
In recent years, in order to cope with an increase in chip size of the semiconductor element, a step-and-repeat type, so-called stepper, which sequentially prints the mask patterns and moves them in the step manner is shifting to a step-and-scan type exposure apparatus which performs scanning and exposure while keeping the mask and wafer in the synchronized state and sequentially moves them to the following shot. This step-and-scan type exposure apparatus has a slit-like exposure field and can accordingly increase the exposure area without increasing the size of the projection optical system.
As the resolving power increases by increasing the NA of the projection optical system and decreasing the wavelength of the light source described above, an exposure amount must be given to the photosensitive material (photoresist) applied to the wafer at high precision, and an increase in precision in exposure amount control has been made.
FIG. 5 is a conceptual view of an exposure apparatus for exposing a circuit pattern image onto a wafer. Referring to FIG. 5, a beam emitted by an excimer laser 1 is shaped into a predetermined beam shape through a beam shaping optical system 2 and becomes incident on an optical integrator 3 formed by two-dimensionally aligning a plurality of small lenses. The optical integrator 3 forms a secondary source near its exit surface 3a. A light beam from the secondary source is focused by a first focusing lens 4. A blind (not shown) for regulating the illumination range is arranged near a plane perpendicularly intersecting an optical axis including a focal point 6 of the first focusing lens 4. A light beam from the first focusing lens 4 uniformly illuminates the pattern surface of a mask 8 through a second focusing lens 7. The pattern of the mask 8 is reduced and projected by a projection optical system 9 to a wafer 10 coated with a photosensitive material. A half mirror 5 is placed between the first focusing lens 4 and focal point 6 to branch part of the light. The photoelectric conversion surface of a sensor 11 is placed near a focal point 6a of the branched light. Hence, the surface of the wafer 10, the pattern surface of the mask 8, and the plane perpendicularly intersecting the optical axis including the focal point 6 are conjugate, and accordingly, the sensor 11 detects the illuminance at a position equivalent to the pattern surface of the mask 8.
A signal from the sensor 11 is amplified by an amplifier 12 and connected to an integrated exposure amount control unit 13 including a CPU (not shown). The integrated exposure amount control unit 13 is connected to the excimer laser 1 to control oscillation of the laser on the basis of the signal from the sensor 11.
Exposure amount control in the step-and-repeat method in the above arrangement will be described with reference to FIG. 6.
FIG. 6 is a flow chart of exposure amount control done by the CPU (not shown) included in the integrated exposure amount control unit 13. When exposure is started for a certain shot, in step 100, an emission pulse count m is set to 0 and a remaining exposure amount Ja(mxe2x88x921) is set to an initial value (target exposure amount Ja). In step 101, the target exposure amount Ja is divided by a standard exposure amount Js per pulse to calculate a total number P of pulses required for exposure. A remainder L of this division is a theoretical number of insufficient pulses (0xe2x89xa6L less than 1) obtained when exposure is performed under the above conditions.
In step 102, the target exposure amount Ja is divided by the total number P of pulses to calculate a preset energy Je of the first pulse. Theoretically, when exposure is performed with this preset energy Je for the number P of pulses, the target exposure amount Ja is obtained.
In step 103, the preset energy Je for one pulse is set for the excimer laser (excimer laser 1 in FIG. 5) through the integrated exposure amount control unit (integrated exposure amount control unit 13 in FIG. 5). In step 104, a 1-pulse emission instruction is made through the integrated exposure amount control unit (integrated exposure amount control unit 13 in FIG. 5).
In step 105, the emission count of the excimer laser (excimer laser 1 in FIG. 5) is incremented. In step 106, an energy Jt per pulse is detected through the sensor (sensor 11 in FIG. 5) and the amplifier (amplifier 12 in FIG. 5).
In step 107, a current remaining exposure amount Jam is calculated from the previous remaining exposure amount Ja(mxe2x88x921) and the current energy Jt per pulse.
In step 108, the preset energy Je for the next pulse is calculated from the remaining exposure amount Jam, the total number P of pulses, and the emission pulse count m from the start of exposure.
In step 109, the total number P of pulses required for exposure and the emission pulse count m from the start of exposure are compared. If P=m, the flow advances to exposure at the next shot position; if p greater than m, the flow returns to step 103.
In this manner, the steps 103 to 108 are repeated until the emission pulse count m reaches the total number P of pulses.
Regarding exposure amount control of the step-and-scan method, for example, as shown in Japanese Patent Laid-Open No. 7-254559, a method is proposed by the present applicant, in which exposure amount control is performed by adjusting the light amount of a pulse beam to irradiate next in accordance with the integrated light amount of the plurality of pulse beams irradiated already, so that the average of the light amounts of a predetermined number of consecutive pulse beams coincides with the target value. In this method, although the flow chart of the exposure amount control is different from that of FIG. 6 described above, its basic method of calculating the energy of the next pulse on the basis of the measurement value of the sensor 11 shown in FIG. 5 is the same as that of FIG. 6.
When ultraviolet radiation is used as the exposure light source, as described above, after the apparatus is used over a long period of time, ammonium sulfate ((NH4)2SO4), silicon dioxide (SiO2), and the like attach to the surface of the optical element placed in the optical path to considerably degrade the optical characteristics. These materials are generated when ammonia (NH3), sulfurous acid (SO2), an Si compound, and the like contained in the ambient environment cause a chemical reaction upon being irradiated with the ultraviolet rays. Conventionally, in order to prevent this degradation in the optical element, the entire optical path is purged with clean dry air or an inert gas such as nitrogen.
Furthermore, it is known that, in the ArF excimer laser having a wavelength of an ultraviolet range, especially about 193 nm, a plurality of absorption bands of oxygen (O2) are present in the band near this wavelength. When oxygen absorbs light, ozone (O3) is generated to enhance absorption of light, thereby greatly decreasing the transmittance. In addition, as described above, various types of products resulting from ozone attach to the surface of the optical element to decrease the efficiency of the optical system.
Therefore, in the optical path of the exposure optical system of the projection exposure apparatus, e.g., the ArF excimer laser, using far ultraviolet radiation as the light source, a method of suppressing the concentration of oxygen present in the optical path to a low level with a purge means using an inert gas, e.g., nitrogen, is employed.
Generally, in the semiconductor element manufacturing process, a large number of circuit patterns overlap the wafer and are exposed. To form a very fine circuit pattern, as described above, the overlapping precision must also be improved. Factors that cause an overlapping error include an image forming performance including the magnification, distortion, focus, and the like of the projection optical system. Factors that change the image forming performance include a change in the surrounding atmospheric pressure or temperature, a change in temperature of the projection optical system caused by exposure, and the like. These factors cause a change in index in the projection optical system and surface distortion of the optical element to change various types of aberrations, resulting in degradation in the image forming performance.
To correct the image forming performance, a method of controlling the position of a specific lens, a method of controlling the pressure of a specific portion of the projection optical system, and the like are available. Above all, when an excimer laser is used as the illumination light source, a method of correcting the image forming performance by changing the oscillation wavelength of the laser can be employed.
The correction method of changing the oscillation wavelength is more advantageous than other correction methods in that it does not require a precision drive mechanism or mechanical control mechanism, so that it can perform correction comparatively easily.
If, however, the ArF excimer laser is used as the illumination light source, since this laser has oxygen absorption bands near 193 nm, as described above, when the oscillation wavelength is changed, the absorbance midway along the optical path undesirably changes in accordance with the wavelengths. In the optical path of the exposure optical system, the concentration of oxygen present in the optical path is suppressed to a low level with the purge means employing an inert gas, e.g., nitrogen, as described above. However, when a drive operation and wafer exchange are frequently performed, like near the wafer surface, it is very difficult to decrease the oxygen concentration to completely zero, and a certain level of oxygen remains. Other than the place near the wafer, wherever oxygen remains, when the oscillation wavelength is changed, the absorbance also changes. As described with reference to FIG. 5, the sensor 11 can detect the light amount until the half mirror 5. In the optical path after the half mirror 5 and before the wafer 10, if the optical absorbance changes, it is impossible to perform integrated exposure amount control with high precision.
The present invention has been made in view of the problems of the prior art described above, and has as its object to enable integrated exposure amount control with high precision even when the wavelength of the light source is changed, so that a very fine circuit pattern can be transferred well.
In order to achieve the above object, according to the present invention, there is provided an exposure apparatus comprising an exposure optical system having a laser as a light source to irradiate a photosensitive substrate with light from the light source, exposure amount control means for controlling an exposure amount for the photosensitive substrate, wavelength changing means for changing an oscillation wavelength of the laser to a predetermined wavelength, and correction means for correcting a target exposure amount to irradiate the photosensitive substrate in accordance with the oscillation wavelength of the laser.
As a signal representing the oscillation wavelength of the laser, for example, an input to or output from the wavelength changing means can be used.
According to a first aspect of the present invention, in an exposure apparatus comprising an illumination optical system having a laser with an oscillation wavelength of 200 nm or less as a light source to irradiate a photosensitive substrate with light from the laser source, exposure amount control means for controlling an exposure amount for the photosensitive substrate, and wavelength changing means for changing an oscillation wavelength of the laser to a predetermined wavelength, the exposure amount control means corrects the exposure amount for the photosensitive substrate on the basis of an output from the wavelength changing means.
According to the second aspect of the present invention, when the exposure amount control means has an integrated exposure measurement unit for integrating a light intensity signal from the light source, correction means is provided to store and correct information concerning a relationship among an output from the integrated exposure measurement unit, an illuminance of the photosensitive substrate, and the oscillation wavelength, and the output value from the integrated exposure measurement unit is corrected on the basis of an output from the wavelength changing means.
According to the third aspect of the present invention, there is provided a projection exposure apparatus which comprises an illumination optical system having an ArF excimer laser with an oscillation wavelength of about 193 nm as a light source to illuminate a mask drawn with a predetermined pattern with light from the light source, and a projection optical system for reducing and projecting a pattern image on the mask onto the photosensitive substrate, and in which exposure amount control means is arranged to detect light branching midway along the optical path of the illumination optical system, wherein the exposure amount control means corrects the exposure amount for the photosensitive substrate on the basis of an output from the wavelength changing means.
According to the fourth aspect of the present invention, in the first aspect described above, a predetermined portion of an optical path, extending from an exit end of the light source to the photosensitive substrate, is divided into a plurality of blocks, and the apparatus has purge means for filling at least one optical element and the optical path accommodated in each of the blocks with an inert gas.
According to the present invention, even if the oscillation wavelength of the laser source is changed for the purpose of, e.g., correcting the image forming characteristics of the projection optical system, a detection error depending on the wavelength may not be present in the measured exposure amount. Therefore, exposure is always performed with an optimal exposure amount, and a very fine circuit pattern of a semiconductor element and the like can be formed well.